(1) Field
The present invention relates to a method for determining the position of a flux vector of an electric motor which is driven by a variable speed drive and is designed to drive a load. The method is achieved without a speed or position sensor (sensorless), and is based on the detection of an error in the estimated position of the flux vector by using a low-frequency current injection.
(2) Description of the Related Art
To obtain an effective control of the torque of an alternating current electric motor driven by a variable speed drive, the system for controlling the drive requires information relating to the position of the flux vector of the motor (usually of the rotor). This position of the rotor flux vector can be estimated satisfactorily in a closed loop with the aid of a rotor position or speed sensor such as a coder. Nevertheless, such a sensor is relatively costly and is mechanically and electrically sensitive which may in particular cause finalization difficulties.
In the absence of the speed or position sensor, that is to say in open loop, the position of the flux vector is estimated based on modelling of the motor and of the voltage of the motor's stator. This approach is effective when the main frequency of the stator voltage, which is the image of the speed reference applied to the motor, is sufficiently high for the internal electromotive force (emf) induced by the flux to be detected reliably. This is usually the case when the main frequency of the stator voltage is higher than approximately 5-10% of the nominal frequency of the stator, that is to say when the speed applied to the motor is higher than approximately 5-10% of the nominal speed.
However, it is known that the control techniques with no position sensor based or a signal at the main frequency are not very effective at low speed because the induced electromotive force then becomes weak and can totally disappear at zero frequency. The fundamental problem is that, when the induced electromotive force depending on the speed becomes low, its evaluation based on the stator voltage becomes imprecise because of errors in the parameters of the motor model (such as the resistance of the stator). The result is that considerable angular differences are made in the evaluation of the position of flux at low speed and at heavy load, which leads to poor performance in control of the motor.
In other situations, such as the driving of a Permanent Magnet Synchronous Motor (PMSM), even if a position sensor is used, the absolute position of the rotor flux vector must be determined before the motor starts. Also necessary therefore is a method making it possible to detect the position of the rotor magnet flux at zero speed.
To provide reliable detection of the position of the rotor flux vector at low speed without using a sensor, various methods based on the injection of an auxiliary signal may be employed. Usually, the injection of a voltage or of a current into the windings of the stator at a frequency different from the main frequency is used in order to determine the positional information of the rotor flux vector by observing and by analyzing the response in current or in voltage of the stator. One method is called low-frequency harmonic current injection. The aim of this method is to generate small oscillations of torque and of rotor speed capable of inducing detectable voltage oscillations. The term “low frequency” in this context means that the injection frequency (or harmonic frequency) is in the mechanical bandwidth of the mechanical drive system. For example, for motors with a nominal frequency equal to approximately 50 Hz-60 Hz, the typical injection frequency would be of the order of 25 Hz-50 Hz.
The low-frequency current injection method is already employed for the control of an induction motor, see in particular: V.-M. Leppanen, J. Luomi, ‘Speed-Sensorless Induction Machine Control for Zero Speed and Frequency’, IEEE Transactions on Industrial Electronics, Vol. 51, No. 5, October 2004, pp. 1041-1047. This method is also employed for controlling a PMSM, see in particular: S. Wu, Y. Li, X. Miao, ‘Comparison of Signal Injection Methods for Sensorless control of PMSM at Very Low Speeds’, IEEE Power Electronics Specialists Conference, PESC 2007, June 2007 pp. 568-573.
This method makes it possible to determine the components of the stator voltage that are induced by the rotor flux following the controlled current injection, so as to subsequently detect the position of the rotor flux vector. For this, one approach is to use a normal PI (Proportional Integral) regulator which is synchronous with the reference frame (SRF—Synchronous Reference Frame) of the main motor current, also called reference frame (d, q) and which has sufficient bandwidth (that is to say capable of regulating the injected current also), and then to use an equation of the stator voltage to determine the inducted internal electromotive force.
However, such a method requires a knowledge of several parameters of the motor (such as the number of poles, motor inertia) and an additional demodulation process in order to determine the positional error signal of the rotor flux vector. This makes the overall algorithm complex and dependent on these parameters. Moreover, the control of the injection currents is not precise and a steady state error may appear in the regulation of these injection currents.